A great deal of interest has developed in producing biaxial oriented thermoplastic bottles having barrier properties and strength characteristics which make them attractive packaging containers for carbonated soft drink beverage containers as well as producing new thermoplastic compositions suitable for other beverages currently bottled in metal and/or glass containers. It will be readily appreciated that the functionality of such containers can be affected by numerous factors especially in the carbonated drink industry where a small volume increase will cause the gas to migrate from the beverage to the non-liquid space causing a deterioration in the quality of the product.
When producing beverage bottles from polymer compositions such as polyethylene terephthalate (PET) and poly(ethylene-2,6-naphthalene dicarboxylate), (PEN), copolymers or blends thereof, it is important that the polymer be well oriented during stretching. Proper orientation results in uniform material distribution in most areas of the bottle. Free-blowing of thermoplastics, particularly for PET and PEN, is a well known technique used to obtain empirical data on the stretching characteristics of a particular thermoplastic formulation. As used herein the term "free-blowing" or "free-blown" means that a preformed article is blow-molded without using a metal mold to obtain a secondary molded article and is analogous to blowing a rubber balloon. Such data desirably includes hoop and axial stretch ratios as well as other physical properties of the thermoplastic. This data is typically used to design a bottle preform for a bottle mold of a given geometry as well as the formulation which will yield the desired bottle properties.
Free-blowing a bottle from a preform involves heating the preform to a temperature above its glass transition temperature (Tg) and then expanding the preform outside of a mold so that it is free to expand without restriction until the onset of strain hardening. In articles made from a thermoplastic such as PET, the onset of strain hardening can be determined by the appearance of "pearlescence". Pearlescence is a slight hazy or "pearly" appearance caused by microcracks giving a hazy appearance with the sheen of an oyster pearl and is a term commonly used in the PET bottle industry to describe a phenomenon that occurs when the onset of strain hardening is exceeded by about 5%. Ideally, the pressure should be sufficient to cause that a very slight amount of pearlescence in the bottle.
In practice, free-blow conditions, such as heating time and blow pressure, are adjusted in a series of trial-and-error iterations so that the free-blown bottle will exhibit a slight amount of pearlescence. If the blow pressure is properly set for a PET preform, it will continue to expand until all of the PET is oriented to the point that stretching will stop at about the natural stretch ratio, or slightly beyond. For example, the optimum pressure for PET may be determined by starting with a relatively low pressure and going up in small increments until a slight amount of pearlescence occurs, then backing off slightly. Once that pressure is determined, it is held constant for all subsequent testing.
Conventional free-blowing processes for preforms made from polymers containing PEN, copolymers of PEN, and blends of PEN and PET have been found to be unsatisfactory and more difficult. Such materials can have much higher stretch ratios than PET, causing free-blown bottles from PEN/PET blends to be much bigger and thus have thinner sidewalls. Moreover, when free-blowing PEN containing copolymers and/or blends, the elevated pressure needed to start the expansion is often too high for the remainder of the free-blowing process. This excess pressure causes the expansion to exceed the natural stretch ratio by a significant amount which is evident by the amount of pearlescence in the free-blown bottle or, in extreme cases, bursting of the bottle.
U.S. Pat. No. 4,090,394 entitled "PLASTIC BOTTLE TESTING" issued on May 23, 1978 discloses a method and apparatus for producing a stress-strain type curve for evaluating the functionality of a thermoplastic bottle. The process uses a piston and an incompressible fluid to pressurize the bottle and graphically recording the movement of the driven piston and the pressure of the fluid in the bottle.
U.S. Pat. No. 4,139,586 entitled "METHOD OF FORMING ARTICLES FROM THERMOPLASTIC SHEET BY FREE-BLOWING" issued on Feb. 13, 1979 discloses a method of vacuum forming thermoplastic sheet material using a mold where a pressure differential on either side of the sheeting is formed to effect free-blowing of the molded article.
U.S. Pat. No. 5,365,792 entitled "METHOD OF DETERMINING STRETCH CHARACTERISTICS OF THERMOPLASTIC ARTICLES" issued Nov. 22, 1994 discloses a method and apparatus for determining the stretching characteristics of a thermoplastic article, such as a bottle preform, by heating the preform to at least its glass transition temperature and stretching the bottle preform using air pressure to the onset of strain hardening then correlating the stretched dimension with the selected physical property.
The above methods and apparatus have proven to be unsatisfactory since they provide inadequate information for purposes of evaluating the functionality of PEN containing thermoplastic bottles. Thus, there is a need in the art for providing a rapid means of determining the functionality of a thermoplastic container and especially for biaxially oriented thermoplastic bottles containing PEN.